Projection exposure apparatus

ABSTRACT

In a projection exposure apparatus wherein a stage is moved in the direction of optic axis when a mask formed with a predetermined pattern is projected by a shutter onto a photosensitive substrate placed on the stage through a projection optical system, the operations of control means for the shutter and control means for the stage are interlocked with each other on the basis of the operational characteristic of the shutter and the operational characteristics (particularly the speed characteristic) of the stage so that the distribution of the existence probability with respect to movement of the photosensitive substrate from the opening operation starting point of time till the closing operation completing point of time of the shutter, with respect to the direction of the optic axis, may assume substantially equal maximum values at at least two locations in the direction of the optic axis.

This is a continuation-in-part of application Ser. No. 946,013 filedSep. 15, 1992, which is a continuation of application Ser. No. 820,244filed Jan. 14, 1992 (now abandoned), which is a continuation-in-part ofapplication Ser. No. 709,278 filed Jun. 3, 1991 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for projecting and exposing asemiconductor circuit pattern, a liquid crystal display element patternor the like onto a photosensitive substrate.

2. Related Background Art

In a projection exposure apparatus of this kind, the exposure of areticle pattern has heretofore been effected with the surface of aphotosensitive substrate (a semiconductor wafer or a glass plate havinga resist layer applied thereto) disposed on the best imaging plane of aprojection optical system (a plane conjugate with the reticle).

However, the area exposed once on the wafer (the shot area) is of theorder of 15 mm square to 20 mm square, and if the surface of the waferis minutely curved in that area or there is unevenness of the order ofseveral μm in the surface structure, there will appear in the shot areaa portion which exceeds the depth of focus of the projection opticalsystem. This is because the depth of focus of the projection opticalsystem is only ±1 μm or so on the image side (the wafer side).

So, a method whereby exposure can be effected at an effectively widedepth of focus even in an exposure apparatus having a projection opticalsystem of a small depth of focus has been proposed in U.S. Pat. No.4,869,999. In the method disclosed in this patent, a wafer is moved totwo or three points in the direction of the optic axis of the projectionoptical system and one and the same reticle pattern is multiplexlyexposed at each point. In this method, two points distant in thedirection of the optic axis are made nearly as wide as the depth offocus ±ΔZ of the projection optical system, thereby enlarging theactually effective depth to the order of 1.5-3 times.

In addition to the method as described in the above-mentioned patentwherein the wafer is positioned at each of multiple points in thedirection of the optic axis and exposure is repeated, there has beenproposed a method whereby the wafer is continuously moved (or vibrated)in the direction of the optic axis during an exposure operation for oneshot area.

A semiconductive integrated circuit is manufactured by the steps of filmformation, pattern transfer, etching, etc. being repeated several to tenand several times. Therefore, in some cases, portions in which filmcorresponding to several layers is laminated and portions in which nofilm is laminated are mixedly present on the surface of a wafer which isin the process of forming an integrated circuit. The thickness of alayer in the film is of the order of 0.1 μm to 1 μm, and the leveldifference on the wafer surface (in one shot area) may be of the orderof several μm at greatest. On the other hand, the depth of focus of theprojection optical system is generally expressed as ±λ/2·NA², where λ isthe wavelength of illuminating light for exposure, and NA is the numeralaperture of the image plane side of the projection optical system. Inthe latest projection optical systems, λ=0.385 μm (the i-line of amercury lamp) and NA≃0.5, and the depth of focus ΔZ in this case isabout ±0.73 μm.

Accordingly, when as in the prior art, exposure is effected with a waferfixedly disposed on the best imaging plane of the projection opticalsystem, both the top and bottom of the level difference on the waferbecome distant in the direction of the optic axis by more than the depthof focus of ±0.73 μm relative to the best imaging plane (the best focusplane) and thus, image formation becomes impossible.

Also, according to a method whereby as in the prior art, exposure iseffected plural times with a wafer positioned at multiple pointsseparate in the direction of the optic axis, it is possible to cope witha level difference of several μm on the wafer, but a shutter system mustbe driven to stop and resume exposure at multiple points in thedirection of the optic axis, and this has led to the problem that thewafer treating ability per unit time is greatly reduced under theinfluences of the driving of a drive stage (Z stage) in the direction ofthe optic axis (Z direction) of a wafer holder, the positioningoperation characteristic and the opening and closing of the shutter.

So, an example of the prior-art exposure method will hereinafter bedescribed with reference to FIGS. 1A-1C of the accompanying drawings.FIGS. 1A and 1B show the time charts of the shutter operation (thevariation in the illumination on a wafer) and the Z stage operation whenmultiplex exposure is effected at two focus positions to one shot area,and FIG. 1C shows the time chart of the shutter operation in a normalmode in which multiplex exposure is not effected.

Here, it is to be understood that during multiplex exposure and duringnormal exposure, the same exposure amount is provided to one shot areaon a wafer. In the case of normal exposure, assuming that the operationtime Ta until the closed shutter is opened and the operation time Tbuntil the shutter is closed from its fully open state are of asubstantially equal value (Tc), a proper exposure amount BV isBV=(Tc+T_(O) ')×IL, where T_(O) ' is the fully open time of the shutter,and IL represents the illumination of the surface of the wafer. Also, inFIGS. 1A and 1C, the ordinate OP. represents the fully open state of theshutter and CL. represents the fully closed state of the shutter.Further, in FIG. 1, time Tst represents the stepping time to the nextshot area of the wafer stage.

On the other hand, in the case of multiplex exposure, the first exposureis effected with the Z stage set at a position +Z₁ upwardly distant fromthe best focus plane Z₀ as shown in FIG. 1B, whereafter the Z stage isre-set at a position -Z₁ downwardly distant from the plane Z₀ during atime T_(Z), and then the second exposure is effected.

The operational characteristics (rising and falling) of the shutter donot vary as long as one and the same exposure apparatus is used andtherefore, the first exposure time is Ta+Tb+T_(O), where T_(O) is thefully open time of the shutter, and if the first exposure amount isabout one half of the proper exposure amount BV, the fully open timeT_(O) is defined as follows (but BV/IL=Tc+T_(O) '): ##EQU1##

Thus, as is apparent from Figures assuming that Ta=Tb=Tc, in the case ofnormal sensitization, the overall treatment time which gives the properexposure amount BV to each shot area on the wafer is ps

    (2Tc+T.sub.O '+Tst)×the number of shots              (1)

and in the case of multiplex (two times) exposure, said over alltreatment time is

    {2(2Tc+T.sub.O +T.sub.Z +Tst}×the number of shots.   (2)

Substituting T_(O) =1/2(T_(O) '-Tc) for this expression (2) andrearranging it,

    (3Tc+T.sub.O '+T.sub.Z +Tst)×the number of shots.    (3)

So, comparing expressions (1) and (3) with each other, it will be seenthat in the case of multiplex exposure, the time becomes longer by(Tc+T_(O)) during each shot.

In the present-day exposure apparatuses (steppers), the time Tc is 10-30mSec. and the time T_(Z), although it differs depending on the strokesof positions +Z₁ and -Z₁, is of the order of 20-50 mSec. Therefore, thetime becomes longer by the order of 30-80 mSec during each shot, andassuming that there are 100 shot areas on a wafer, the treatment of onewafer may become longer by 3 to 8 sec.

As described above, the prior-art method has suffered from a problem isthe throughput of wafer treatment.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-noted problemand the object thereof is to provide a projection exposure apparatus inwhich the substantial depth of focus is enlarged and at the same time,the reduction in throughput is decreased and in addition, thecontrollability of exposure amount is secured.

To achieve this object, the present invention is constructed as follows.

A Z stage is moved in the direction of the optic axis during exposure ofa photosensitive substrate.

Interlocking means for interlocking the operations of control means fora shutter and control means for the Z stage is provided so that on thebasis of the operational characteristics of the shutter and theoperational characteristics (particularly the speed characteristic) of amovable stage (Z stage), the distribution of the existence probability(i.e., the existence time per unit length in the Z direction) obtainedunder the movement of said substrate photosensitive substrate (a resistlayer on a wafer) from a point of time at which the shutter starts to beclosed till a point of time at which the shutter is completely closed,with respect to the direction of the optic axis may assume substantiallyequal maximum values at least two locations in the direction of theoptic axis.

In the present invention, there is adopted a system for moving thephotosensitive substrate such as the wafer and the projected imagerelative to each other in the direction of the optic axis while theshutter is open, so as to endow the opening-closing controlcharacteristic of the shutter and the movement control characteristic inthe direction of the optic axis with a special relation.

The manner of exposure by the prior-art multiplex exposure system willnow be described with reference to FIG. 2 of the accompanying drawings.

FIG. 2, (A), (B) and (C) show the manners of multiplex exposure at focuspositions -Z₁, Z₀ and +Z₁, respectively, and the low portion, the middleportion and the high portion of the pattern level difference on a waferare represented as (A), (B) and (C), respectively. The pattern to besubjected to multiplex exposure exists as a hole pattern (a white minuterectangle) formed on a reticle. The intensity distribution of theprojected image of this hole pattern on the wafer is shown by Fmn, andm=1 represents the focus position -Z₁, m=2 represents the focus positionZ₀, m=3 represents the focus position +Z₁, and n=1, 2, 3 represents thenumber of times (order) of exposure.

When at first, the first exposure is effected with the best imagingplane of a projection lens focused on the low portion (the position -Z₁)of the pattern level difference on the wafer, a sharp intensitydistribution F₁₁ is imaged on the low portion, but toward the middle andhigh portions, the intensity distribution F₁₁, is suddenly deteriorated(decreased in peak value and increased in width). When the secondexposure is then effected with the best imaging plane focused on themiddle portion (the position Z₀) of the pattern level difference, asharp intensity distribution F₂₂ is imaged on the middle portion, but adeteriorated intensity distribution F₂₂ ' appears on each of the lowportion and the high portion. Likewise in the third exposure, focusingis effected on the high portion (+Z₁) of the pattern level differenceand therefore, a sharp intensity distribution F₃₃ is obtained in thehigh portion, and toward the middle portion and the low portion, adeteriorated intensity distribution F₃₃ ' is obtained.

When three times of exposure is thus finished, sharp image distributionsF₁₁, F₂₂ and F₃₃ are obtained once in any of the low portion, the middleportion and the hight portion of the pattern level difference.

The integrated quantity of light of the intensity distributions F₁₁, F₂₂' and F₃₃ ' is provided to the resist of the low portion, as shown inFIG. 2(A) , and the distribution of the integrated quantity of light isas shown in FIG. 2 (D). In FIG. 2 (D), the level Eth indicated by abroken line is the exposure amount necessary to remove positive resist(form a hole pattern). Likewise the integrated quantity of light of theintensity distributions F₂₂, F₁₁ ' and F₃₃ ' is provided to the resistof the middle portion of the pattern level difference, as shown in FIG.2(E), and the integrated quantity of light of the intensitydistributions F₃₃, F₁₁ ' and F₂₂ ' is provided to the resist of the highportion of the pattern level difference, as shown in FIG. 2(F). A goodimage intensity distribution of a hole pattern is provided to any ofthese three level difference portions and as a result, the apparentenlargement of the depth of focus is accomplished over the width of 2Z₁from the high portion to the low portion of the level difference.

Besides the above-described method of effecting multiplex exposure whilediscretely varying the focus position, the effect of enlarging the depthof focus is also obtained by a method of effecting exposure whilecontinuously moving a wafer in the direction of the optic axis. However,according to the comfirmation through experiments or the like, it hasbeen found that the expected enlarging effect cannot be attained even ifa wafer is moved (or vibrated) at random in the direction of the opticaxis during the exposure of the wafer (during the opening of theshutter).

Assuming that as a generally conceivable moving system, a Z stageholding a wafer thereon is driven at a uniform speed, the timedistribution of the best focus image with respect to the direction ofthe optic axis, i.e., the so-called existence probability, is shown inFIG. 3 of the accompanying drawings. In FIG. 3, the ordinate representsthe focus position (the position of the wafer in the direction of theoptic axis) and the abscissa represents the existence probability (i.e.,the time per unit length in the direction of the optic axis with whichthe best focus image exists). In the previously described three-pointmultiplex exposure, the existence probability has assumed other valuesthan zero only at the three focus positions +Z₁, Z₀ and -Z₁, but in theuniform speed movement of the Z stage, a constant existence probabilityis assumed over the entire range of ±Z₁, and besides the enlargingeffect for the depth of focus, the disadvantage of the aggravatedcontrast of the exposed resist pattern presents itself remarkably.

Also, in the technique of vibrating the wafer in the direction of theoptic axis during the exposure of the wafer, the enlarging effect forthe depth of focus may not be sufficiently expected depending on theamplitude, frequency and waveform of the vibration and the exposuretime.

So, in the present embodiment, the continuous movements of the wafer andthe projection image plane in the direction of the optic axis arecontrolled so that the existence probability may be maximized at thefocus positions +Z₁ and -Z₁, in other words, two points distant fromeach other by an amount roughly corresponding to the width of the depthof focus of the projection lens and that in the intermediate portionbetween the two points, the existence probability may be suppressed lowto such an extent that will not cause the deterioration of the contrastof the exposed resist pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are times charts showing the driving sequences of the Zstage and shutter in the multiplex exposure system according to theprior art.

FIG. 2 illustrates the variation in the distribution of quantity oflight of a hole pattern when multiplex exposure is effected at threepoints.

FIG. 3 shows the distribution of exposure amount (existence probability)when exposure is effected while the Z stage is moved at a uniform speed.

FIG. 4 shows the construction of a projection exposure apparatusaccording to an embodiment of the present invention.

FIG. 5 is a block diagram showing the construction of the main controlsystem in FIG. 4.

FIGS. 6A-6C are graphs showing an example of the illumination variationcharacteristic resulting from the opening-closing of a shutter and thedriving characteristic of the Z stage.

FIG. 7 is a graph showing the distribution of existence probability in Zdirection (the direction of the optic axis) obtained under the movementconditions of FIG. 6.

FIG. 8 is a graph showing the distribution when the exposure amount isconcentrated in three locations in Z direction.

FIG. 9 is a graph showing the simulated distribution of existenceprobability.

FIG. 10 is a diagram showing the condition to simulate the distributionof light amount of a contact hole image.

FIG. 11 shows the construction of a projection exposure apparatus whichis the premise of a fourth embodiment.

FIG. 12 is a block diagram showing a focus detecting system of theoblique incident light type and a control system for the Z stage.

FIG. 13 is a functional block diagram showing the constructions of anexposure control portion and a main control portion.

FIGS. 14A and 14B are graphs showing the shutter characteristic and themovement characteristic of the Z stage.

FIG. 15 is a graph showing an example of the weight ratio of theexposure amount in the direction of the optic axis.

FIG. 16 shows the waveform of a synchronous detection output signal.

FIG. 17 shows an example of the construction of a driving circuit forthe Z stage.

FIG. 18 a flow chart showing an example of the operation.

FIG. 19 is a flow chart showing an example of the operation.

FIG. 20 is a shot arrangement map showing the exposure sequence on awafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows the construction of a projection exposure apparatusaccording to an embodiment of the present invention.

In FIG. 4, illuminating light from a mercury discharge lamp 1 iscondensed on a second focus by an elliptical mirror 2. A rotary shutter3 having a plurality of blades is disposed near the second focus forrotation by a motor 4. The illuminating light condensed on the secondfocus and transmitted through the shutter 3 enters an optical integrator6 including an input lens 5, a fly-eye lens, etc. A plurality ofsecondary light source images are formed on the exit surface side of theoptical integrator 6, whereby a surface light source is made. Theilluminating lights from the respective secondary light source imagesare reflected by a mirror 7 having a slight transmittance (e.g. of theorder of 10%), whereafter they enter a first relay lens system 8 and areintegrated by the surface of a reticle blind 9, and illuminate the blind9 with a uniformized illumination distribution. The blind 9 is forlimiting the illuminated range on a reticle R, and the opening image ofthe blind 9 is formed on the pattern surface of the reticle R through asecond relay lens system 10, a mirror 11 and a condenser lens 12.

The illuminating light transmitted through the transparent portion ofthe pattern of the reticle R passes through a projection lens PL to awafer W, and the projected image of the reticle pattern is formed nearthe surface of the wafer W. The wafer W is vacuum-attracted onto theholder of a Z stage 14 disposed on an XY stage 13. The Z stage 14 ismoved on the XY stage 13 in the direction of the optic axis of theprojection lens PL by a driving portion (a motor and a tachogenerator)15.

Usually, the Z stage 14 is moved in the direction of the optic axis bythe driving portion 15 being servo-operated under the control of an AFunit 18 which inputs a focus signal from an oblique incident light typeAF sensor comprised of a light projector 16 and a light receiving device17. This is an auto-focus operation effected during normal one-timeexposure.

A photoelectric sensor 19 for detecting the intensity of theilluminating light (the pattern image of the reticle R or the openingimage of the blind 9) passed through the projection lens PL is providedon the XY stage 13.

Now, a photoelectric sensor 20 for detecting the intensity of part ofthe illuminating light passed through the shutter 3 is disposedrearwardly of the mirror 7, and the photoelectric signal of thephotoelectric sensor 20 is input to an integrator circuit 21 whichdigitally effects the integration of the quantity of light. Theintegrator circuit 21 outputs to a main control system 100 a signal ILobtained by the photoelectric signal being amplified by a predeterminedamount. The signal IL is proportional to the illumination on the reticleR or the illumination on the wafer W. Further, the integrator circuit 21includes a comparison circuit which outputs the closing signal Sc of theshutter 3 from the main control system 100 when the value of a presetproper exposure amount and the integrated value of the quantity of lightcoincide with each other. This closing signal Sc is sent to a shutterdriver 22, whereby the motor 4 is rotated by a predetermined amount tothereby close the shutter 3. Also, the driver 22 exchanges an openingcommand for the shutter 3, a shutter status signal, etc. between it andthe main control system 100.

In the case of normal one-time exposure, automatic exposure amountcontrol is effected by the loop of the photoelectric sensor 20,integrator circuit 21, shutter driver 22 and motor 4. On the other hand,the mercury discharge lamp (hereinafter simply referred to as the lamp)1 has its supplied electric power controlled by a lamp control unit 23.In recent years, in steppers, in order to enhance the throughput ofexposure, there has been adopted a flash exposure system whereby thesupplied electric power to the lamp 1 is increased to about twice therated electric power only when the shutter 3 is open. The lamp controlunit 23 is given a changeover command for the normal exposure system orthe flash exposure system from the main control system 100, and in thecase of flash exposure, it increases the supplied electric power to thelamp 1 in response to the start of the opening of the shutter 3.

The XY stage 13 in FIG. 4 steps so as to move respective ones of theplurality of shot areas on the wafer W to just beneath the projectionlens PL, but this is not directly related to the present invention andtherefore, the driving portion and the control system for the XY stage13 are not shown.

The functional blocks of the main control system 100 relating to theessential portions of the present invention will now be described withreference to FIG. 5. The function of each block in FIG. 5 is achieved bythe hardware of an electric circuit or the software of a microcomputeror the like.

In FIG. 5, an analog-digital converter ADC 110 receives as an input theamplified photoelectric signal IL from the photoelectric sensor 20, andconverts the level of the signal IL into a digital value during eachpredetermined sampling time. The converted digital value is stored in amemory (RAM) 111 in order of address. The address value of the RAM 111is generated by a counter 112, and this counter 112 counts the pulsenumber of a Clock pulse CKP made by a clock generator 113 only while theclock pulse CKP passes through a gate 114. The opening and closing ofthe gate 114 are changed over by a signal CS, from a microprocessor (μP)150.

This microprocessor μP 150 is connected to both of the AF unit 18 andthe shutter driver 22 in FIG. 4 through bus lines DB₁ and DB₂, andeffects the exchange of data necessary for control. A data input portion(or a man machine interface) 120 receives as inputs commands and datafrom the operator, and main commands are a changeover command formultiplex exposure or nominal exposure and an exposure starting command,and main data are the proper (target) exposure amount per shot and themovement width (2Z₁) of the Z stage 14 during multiplex exposure. A Zstage characteristic data portion 130 stores therein chiefly the data ofthe movement speed characteristic of the Z stage 14 (the maximum speedvalue, the acceleration value, etc.). The data of this speedcharacteristic can be easily found by converting the output value fromthe tachogenerator provided in the driving portion 15 for the Z stage 14into a digital value by an A/D converter during each predeterminedsampling time, and reading the digital value into the memory in themicroprocessor μP 150 through the bus line DB₁, and thereafter analyzingit by the microprocessor μP 150. The data thus found by themicroprocessor μP are stored in the Z stage characteristic data portion130, but the Z stage may sometimes differ in its speed characteristicbetween a case where it is moved from below to above and a case where itis moved from above to below and therefore, it is preferable to find andstore speed characteristics with respect to the both cases.

The microprocessor μP 150 controls the stoppage of the outputting of thesignal CS₁ to the gate 114 on the basis of the inputting of the closingsignal Sc of the shutter 3. Further, the microprocessor μP 150 outputsvarious warnings to the operator through a bus line DB₃. As one of thewarnings, it is reported that it is difficult to effect multiplexexposure under exposure conditions input to the data input portion 120and accordingly, it is necessary to revise some of the conditions.

The basic operation of the present embodiment will now be described.

Let it be assumed that the shutter 3 in FIG. 4 is in its closed state,the lamp control unit 23 is set to the normal exposure system, and theintegrated value of the quantity of light in the integrator circuit 21is reset to zero.

Prior to exposure, the microprocessor μP 150 sets the value of theproper exposure amount input to the data input portion 120 in one inputof the comparison circuit of the integrator circuit 21 and also outputsan opening command for the shutter 3 to the shutter driver 22 throughthe bus line DB₂ in the one-time exposure mode with the wafer W beingabsent under the projection lens PL. Thereby, the shutter driver 22controls the closing of the shutter 3 so that the exposure amount on thereticle R or the wafer W may be a proper value. At this time, themicroprocessor μP 150 delivers the signal CS₁ to the gate 114simultaneously with the generation of the opening command for theshutter, and starts the counting up by the counter 112. The counting upby the counter 112 is stopped by the outputting of the signal CS₁ beingdiscontinued in a predetermined time after the shutter closing signal Scis produced. That predetermined time is made somewhat longer than theoperational delay (electrical and mechanical delay) during the closingof the shutter.

When in this manner, dummy exposure is effected in the one-time exposuremode under conditions under which proper exposure is obtained for apredetermined wafer, an illumination variation characteristic P (t) onthe reticle is obtained in the RAM 111 in FIG. 5 as shown, for example,in FIG. 6A.

The ordinate of this characteristic P (t) represents the illuminationvalue, and the absciss- a represents time and it is to be understoodthat, at a time t_(SO), the signal CS₁ in FIG. 5 is produced and thecounting up by the counter 112 is started. Also, a time t_(SC) in FIG.6A represents a point of time at which the shutter closing signal Sc isproduced from the integrator circuit 21.

Further, a time t₀ on the characteristic P(t) represents a point of timeat which the illuminating light passed through the shutter 3 actuallybegins to be applied to the reticle R, and has a substantially constantdelay with respect to the time t_(SO) when the signal CS₁ which is anexposure starting command is produced. Likewise, the time t₅ when theapplications of the exposure light to the reticle R is completelydiscontinued has a substantially constant delay with respect to the timet_(SC) when the signal Sc is produced.

As is apparent from FIG. 6A, the total exposure amount (proper exposureamount) Eu provided to the wafer W is expressed as follows with k as aconstant: ##EQU2##

It is because the operational speeds of the shutter 3, the motor 4, thedriver 22, etc. are finite that the characteristic P(t) of FIG. 6A doesnot become rectangular with respect to time. However, thereproducibility of the characteristic P(t) is sufficiently good, andduring multitime exposure (during the opening and closing of theshutter), the relation of the characteristic P(t) is reproducedsubstantially completely.

The microprocessor μP 150 shown in FIG. 5 calculates delay times (t₀-t_(SO)) and (t₅ -t_(SC)) in the characteristic P(t) and theacceleration of the variation in illumination from the shutter openingoperation starting time t₀ till the shutter closing completion time t₅,on the basis of the data stored in the RAM 111, and stores them in thememory in the microprocessor μP 150.

Now, FIG. 6B shows an example of the characteristic V(t) when during theexposure operation, the Z stage 14 is moved at a special speed patternin the direction of the optic axis on the basis of the speed data storedin the Z stage characteristic data portion 130. Here, the Z stage 14begins to be moved at a low speed v₁ at a time t_(S) before the time t₀when the shutter opening command is generated, and keeps the speed v₁from the time t₀ till a time t₁ which is a predetermined time after thetime t₀, and accelerates to a speed v₂ approximate to the highest speedfrom after the time t₁. Then, the Z stage decelerates again to a lowspeed v₃ (substantially equal to v₁) from after a time t₃ after the Zstage is stabilized at the speed v₂, and is maintained at the speed v₃until the closing of the shutter is completed.

FIG. 6C shows the position characteristic Z(t) of the Z stage 14 in thedirection of the optic axis when the Z stage 14 is moved at thecharacteristic V(t) of FIG. 6B. In FIG. 6C, a position ZC is a positionat which the pattern projection image of the reticle R coincides withthe surface of the wafer as the best image.

In the present embodiment, the speed of the Z stage 14 is brought intothe highest state near the position at which the best image planecoincides with the surface of the wafer, and is kept at the lowestpossible speed during the opening and closing operations of the shutter3. That is, during one shutter opening operation, the Z stage 14 ismoved while being accelerated and decelerated at a predeterminedcharacteristic.

Therefore, it is necessary that driving conditions such as theacceleration condition and the deceleration condition of the Z stage 14be determined before exposure is started.

Now, of course , the step of introducing the illumination characteristicP(t) into the RAM 111 and calculating various data may be suitablyexecuted plural times and the result may be averaged. Also, in theprevious example, the opening-closing of the shutter 3 is controlled sothat a proper exposure amount may be obtained from first by theintegrator circuit 21, but in the case of a mode in which theopening-closing of the shutter 3 is controlled by time, the set shuttertime can be adjusted until a proper exposure amount is obtained, andafter the proper exposure amount is obtained, the characteristic P(t) ofFIG. 6A can be stored in the RAM 111. In order to confirm whether properexposure is obtained on the wafer surface side (the image side of theprojection lens PL), use may also be made of a photoelectric sensor 19provided on the XY stage 13. Further, the characteristic P (t) to bestored in the RAM 111 may be made from the photoelectric signal of thephotoelectric sensor 19 obtained through a processing system 24.

Description will now be made of a method of determining the movementconditions of the Z stage 14 in the present embodiment. Supposing theactual exposure operation, the wafer W is at a position Zs before theexposure is started, and the wafer W starts movement in the Z direction(the direction of the optic axis) from here. Now, as shown in FIGS.6A-6C, the Z stage 14 at the position Zs starts to be driven at a timet_(s), and is uniformly moved at a low speed v₁. The shutter openingcommand (signal CS₁) is output at a time t_(SO) so that the shutter 3may begin to be opened after the speed of the Z stage 14 is stabilizedat v₁. Thereupon, at a time t₀, exposure to the wafer W is started andat that time, the Z stage 14 has arrived at a position Z₀. The uniformmovement at the speed v₁ is continued until a time t₁, i.e., until theposition of the wafer W in the Z direction comes to an accelerationposition Z₁, whereafter the Z stage 14 is again accelerated at anacceleration a and at a time t₂, it reaches a speed v₂ (thesubstantially highest speed). Accordingly, the acceleration a isexpressed as follows:

    a=(v.sub.2 -v.sub.1)/(t.sub.2 -t.sub.1)                    (2)

Also, the positions of the Z stage 14 at the times t₁ and t₂ are Z₁ andZ₂. From the time t₂ till a time t₃, the Z stage is uniformly moved atthe speed v₂ and on its way, the Z stage passes through the best focusposition ZC for the wafer W. When at the time t₃, the Z stage comes to aposition Z₃, the Z stage is decelerated at an acceleration β, and at atime t₄ (a position Z₄), it is dropped to a low speed v₃. At this time,the acceleration β (β<0) is expressed as follows:

    β=(v.sub.3 -v.sub.2)/(t.sub.4 -t.sub.3)               (3)

From the time t₄ till a time t₅ (from the position Z₄ to a position Z₅), the wafer W is uniformly moved at the speed v₃, and at a time t_(SC),the shutter closing command (signal Sc) is output.

In the present embodiment, exposure amounts E₁ and E₄ provided to thewafer W at the positions Z₀, Z₁, Z₄ and Z₅ and between the positions Z₀-Z₁ and between the positions Z₄ -Z₅ greatly contribute to theenlargement of the depth of focus.

The estimation of these exposure amounts E₁ and E₄ is pre-calculated bythe microprocessor μP 150 in FIG. 5, and in this calculation, conditionsmay be automatically determined by the use of the amount of depth offocus (±DOF) of the projection lens PL itself and the proper exposureamount (total exposure amount) Eu input from the data input portion 120,but an operation processed under conditions uniquely set by the operatoris also regarded as possible.

Now, the conditions are automatically determined as an example asfollows by the microprocessor μP 150:

    Z.sub.0 =+DOF, Z.sub.1 =+0.8×DOF

    Z.sub.5 =-DOF, Z.sub.4 =-0.8×DOF

    E.sub.1 =0.35×Eu, E.sub.4 =0.35×Eu

As is clear from this, the exposure amount provided to the wafer W whilethe Z stage 14 is moved from the position Z₁ to the position Z₄ is about30% of the total exposure amount Eu.

The numerical values (parameters) shown above by way of example arestandard values to the last, and can be suitably changed by theoperator. However, if the parameters are changed at random, it does notalways follow from various limitations (such as the speed and movementstroke of the Z stage and the illumination) that the conditions areproperly set. In that case, as will be described later in detail, awarning is generated through the bus line DB₃ to inform the operationthat the proper setting of the conditions is impossible by theparameters changed by the operator.

Now, when the exposure amount E₁ is determined in accordance with theabove-described parameters, the time t₁ when the exposure amountintegrated from the exposure starting time t₀ (the integrated value)becomes E₁ can be found from the relation of the following equation (inwhich k₁ is a constant), on the-basis of the illumination variationcharacteristic P(t) stored in the RAM 111: ##EQU3##

When the time t₁ is thus found, the speed v₁ at which the Z stage is tobe moved from the position Z₀ to the position Z₁ at the time (t₁ -t₀)can be found from the following equation:

    v.sub.1 =(Z.sub.1 -Z.sub.0)/(t.sub.1 -t.sub.0)             (5)

Here, the spacing between the positions Z₀ and Z₁ may preferably be assmall as possible relative to the depth of focus DOF (ZC-Z₀, Z₅ -ZC) ofthe projection lens PL, and as an example, may desirably be 1/3 or lessof the value DOF.

When the exposure amount E₄ is determined in a similar manner, the timet₄ when the exposure amount (integrated value) provided to the wafer Wfrom the time t₄ till the time t₅ becomes E₄ is inversely calculatedfrom the relation of the following equation (in which k₂ is a constant)on the basis of the illumination variation characteristic P(t): ##EQU4##

Here, the time t₅ is known, and is already prescribed on thecharacteristic P(t) stored in the RAM 111.

When the time t₄ is thus found, the speed v₃ at which the Z stage is tobe moved from the position Z₄ to the position Z₅ at the time (t₅ -t₄)can be found from the following equation:

    v.sub.3 =(Z.sub.5 -Z.sub.4)/(t.sub.5 -t.sub.4)             (7)

Here, the spacing between the positions Z₄ and Z₅ may also desirably be1/3 or less of the depth of focus DOF of the projection lens PL. By theabove-described calculations, the times t₀ -t₁, the times t₄ -t₅ and thespeeds v₁ and v₃ are determined.

From what has been described above, it will be seen that when the totalexposure time (t₅ -t₀) is Tt, the movement of the Z stage 14 from theposition Z₁ to the position Z₄ (the acceleration and deceleration rangefrom the time t₁ till the time t₄) should be effected during

    Tt-(t.sub.1 -t.sub.0)-(t.sub.5 -t.sub.4)=Tαβ    (8)

At this time, from the acceleration and deceleration characteristics ofthe Z stage 14, the time (t₂ -t₁) and the time (t₄ -t₃) aresubstantially constant values and thus, when the total of the time (t₂-t₁) and the time (t₄ -t₃) becomes greater than the-time Tαβ, thecontrol of the Z stage becomes unstable and the expected depth enlargingeffect cannot be attained.

When it is confirmed that

(t₂ -t₁)+(t₄ -t₃)≦Tαβ, the microprocessor μP 150 finds the relationamong the acceleration discontinuing time t₂ and the: decelerationstarting time t₃ of the Z stage 14 and the speed v₂. Here, the distancefrom the position Z₁ to the position Z₄ is represented by the followingequation: ##EQU5##

Further, the accelerations a and β are known from equations (2) and (3)and therefore, this equation (9) can be rewritten as follows: ##EQU6##

In this equation (10), the times t₁ and t₄ and the speeds v₁ and v₃ haveall been previously determined and therefore, equation (10) becomes arelational expression having v₂, t₂ and t₃ as parameters.

Accordingly, if the speed v₂ and the time t₂ are suitably determined,the time t₃ will be correspondingly determined and the positions and thedriving conditions of the Z stage at the respective times duringexposure (t₀ -t₅) will be accurately determined.

In the case of the present embodiment, the best focus position ZC liessubstantially at the midpoint between the positions Z₁ and Z₄ andtherefore, as shown in FIG. 6C, the position characteristic Z(t) becomesa curve substantially point-symmetrical about the position ZC.

Also, in determining the driving conditions of the Z stage 14, theaccelerations a and β of the Z stage may be fixed at predeterminedvalues in advance on the basis of equations (9), (2) and (3) and thetimes t₂ and t₄ may be determined in conformity with the value of thespeed v₂ from equations (2) and (3), whereby the driving conditions canlikewise be found accurately.

Since in the manner described above, the movement conditions includingthe movement speed of the wafer W in the Z direction during exposure andthe Z position are determined, exposure may thereafter be effected inaccordance with the determined conditions, whereby between the positionsZ₀ to Z₁, the exposure amount E₁ is accurately provided to the wafer W,and between the positions Z₄ to Z₅, the exposure amount E₄ is accuratelyprovided to the wafer W. At the same time, the total exposure amount Euis also controlled accurately. Also, if the acceleration a between thetimes t₁ to t₂ and the acceleration β between the times t₃ to t₄ arecontrolled to a=-β, the existence probability of the wafer at thepositions Z₁ to Z₄ with respect to the Z direction will become ZC≃(Z₁+Z₄)/2 substantially-symmetrically about the best focus position ZC, andthis will greatly contribute to the effect of enlarging the depth offocus.

The initial position of the Z stage 14 has been shown as Z₅ butalternatively, the initial position may be a position above Z₅ and withthe progress of exposure, the Z stage may be moved reversely to Z₄, Z₃,. . . , Z₀.

Also, the Z position of the Z stage during exposure can be detected bythe AF unit 18 shown in FIG. 4, but if the movement width (2·DOF) of theZ stage during exposure is great, the Z position may exceed thedetection range of the AF sensor. In such case, the focus signal cannotbe obtained near the positions Z₁ and Z₄ and therefore, design can bemade such that the positional information is obtained from apotentiometer or a position sensor which monitors the amount of drivingof the Z stage. In connection with this, for example, the time when atthe best focus position ZC, the focus signal assumes a staterepresentative of in-focus (for example, the zero point) is checked upwhile the level of the surface of the wafer W is being monitored by anAF sensor, and whether this time is substantially at the midpointbetween the times t₂ and t₃ is judged, whereby whether the exposureoperation for one shot has been effected well can be roughly known.

Now, FIG. 7 graphically shows the existence probability of the wafer(or, concentricity of the exposure amount) with respect to the Zdirection when exposure is effected at the characteristics shown inFIGS. 6A, 6B and 6C. From the fact that the peaks of the existenceprobability exist near the positions Z₀ -Z₁ and near the positions Z₄-Z₅, the enlargement of the depth of focus can be achieved. In thepresent embodiment, the exposure amount cannot be made zero at thepositions between the peaks, e.g. Z₂ -Z₃. This is because the Z stage iscontinuously moved. To most enhance the effect of enlarging the depth offocus, it is necessary that the existence probability shown in FIG. 7 isprovided and the exposure amount (E₁) at the positions Z₀ -Z₂ and theexposure amount (E₄) at the positions Z₃ -Z₅ be made substantially equalto each other, but in the present embodiment, the speed characteristicV(t) and the position characteristic Z(t) of the Z stage and theillumination characteristic P(t) during the opening-closing of theshutter are strictly controlled and therefore, the control accuracy ofthe exposure amount is maintained good.

FIG. 8 shows the existence probability of the wafer when the number oftimes of multiplex exposure is three, and represents a case where as thedriving pattern of the Z stage 14 when the shutter is open, the lowspeed uniform movement→acceleration and deceleration→the low speeduniform movement are repeated twice. Peaks exist at three locations withrespect to the position in the Z direction, and the central peak isadjusted to the best focus position ZC, and the peaks on the oppositesides of the central peak are spaced apart from each other by an amountcorresponding to the depth-of--focus width 2·DOF of the projection lensPL. Again in this case, it is more desirable that the exposure amountscorresponding to the three peaks be uniform.

As described above, in the present embodiment, the characteristic P(t)of FIG. 6A is proportionally enlarged in the direction of the time axiswith the deterioration of the lamp intensity and therefore, after onelot of wafers (25 wafers) have been treated, the characteristic P(t)should preferably be re-measured. Also, after the characteristic P(t)has been stored in the RAM 111, it is better to adopt a timer mode inwhich the operation of the shutter 3 is controlled by time, and controlthe time t_(so) -t_(sc) so as to be set as the shutter time.

A second embodiment of the present invention will now be described.

In the second embodiment, when it is known on the basis of a warningsignal obtained through the bus line DB₃ of the microprocessor μP 150that multiplex exposure becomes incomplete in the calculation process,there is given the function of relieving it. Description will first bemade of a state in which the warning signal may be produced.

In most cases, the warning is produced when the time from the openingstart time t₀ of the shutter 3 till the closing completion time t₅ ofthe shutter 3 becomes considerably short as compared with the movementspeed variation pattern of the Z stage 14. For example, when from thelimit of the driving characteristic of the Z stage 14, the time t₁ -t₄becomes so short that the constant high speed range (speed v₂) as shownin 6B does not exist, the range in which a minimum existence probabilityis provided between the positions Z₂ -Z₃ as shown in FIG. 7 becomes nulland the effect of enlarging the depth of focus cannot sufficientlyobtained.

Accordingly, at a point of time whereat it is found in the calculationprocess that good control is impossible, the actual exposure time (t₀-t₅) for the wafer W can be lengthened until good control becomespossible.

In the present embodiment, the exposure time per one shot becomesslightly longer and some reduction in throughput is caused, but thereduction in throughput heretofore experienced can be avoided. For thatpurpose, an ND filter for stepwisely changing the intensity of theilluminating light is removably provided in the illuminating opticalsystem of the projection exposure apparatus. As regards thetransmittance of the ND filter, light may be decreased, for example, atevery 10% and finally to the order of 50%.

As another means, it would occur to mind to reduce the supplied electricpower to the lamp 1 through the lamp control unit 23 shown in FIG. 4. Insuch case, the supplied electric power must be reduced within such arange that the lamp 1 continues to be stably turned on.

In any case, in a state in which after the intensity of the illuminatinglight is reduced by a predetermined amount, the opening-closing of theshutter 3 is again effected so that a proper exposure amount may beobtained, the illumination characteristic P(t) can be again stored inthe RAM 111 and similar calculation can be executed.

While each embodiment has been described with respect to an example inwhich the wafer W is moved in the direction of the optic axis, a similareffect will be obtained by a construction in which a chose air chamberin the projection lens PL is hermetically sealed and the pressure in theair chamber is forcibly adjusted, whereby the focus position of theprojection lens PL itself (the surface conjugate with the reticle) ismoved in the direction of the optic axis. Also, in the case of aboth-side telecentric projection lens, even if a construction is adoptedin which one or more lens elements positioned on the reticle side arefinely moved by a piezo element or the like, the projected image (thebest focus plane) can be parallel-shifted in the direction of the opticaxis.

Next, an example of optimum control at the time when projection exposureis effected with respect to a contact hole pattern will be explained asa third embodiment of the invention. In this embodiment exposure amountin the direction of the optic axis is concentrated in the same way asthat of the foregoing embodiments.

FIG. 9 shows the characteristic of the existence probability simulatedon the basis of the position characteristic Z(t) of the Z stage and isthe same one as shown in FIG. 7. In FIG. 9, the existence probabilityfrom times t₀ to t₅ is shown, and the existence probability is constantin the low speed uniform movement ranges from times t₀ to t₁ and fromtimes t₄ to t₅. For simplifying the explanation below, changes inillumination at the rise and the fall of the illumination characteristicP(t) are neglected, and it is assumed that the illumination is constantover time interval from t₀ to t₅ (where the shutter is fully opened).

Generally, the existence probability is defined as time Δt required forunit displacement ΔZ in the Z direction, and is treated as Δt/ΔZ.

In the case where exposure is effected with respect to the projectionimage of the contact hole pattern under distribution of such existenceprobability, the intensity distribution in the optic axis in the centerof the projection image or in the neighborhood thereof of one contacthole pattern may be considered to take approximately a Gaussiandistribution because the size of the pattern is close to the resolutionlimit of the projection lens. FIG. 10 is the result of simulation on thedistribution of the quantity of light by the contact hole image in thedirection of the optic axis.

The distribution of the quantity of light in the direction of the opticaxis (in the Z direction) may be obtained by convolution of the productof the existence probability characteristic (function) shown FIG. 9 andthe function of the Gaussian distribution. In the case of simulation bya computer, Gaussian distribution curve GP(Z) is so shifted in the Zdirection along the curve of the existence probability that the peak ofthe curve GP(Z) at each point in the Z direction may be located on thecurve of the existence probability, and the Gaussian distribution curvesthus obtained are summed to obtain the light quantity distributionIM(Z).

The width in the Z direction at a level where the intensity of oneGaussian distribution GP(Z) is about 80% of the peak value, correspondsto the effective "depth of focus" width 2·DOF of the projection lens.From the simulation shown in FIG. 10, the light guantity distributionIM(Z) is substantially flat in the movement range on width from Z₀ to Z₅of the Z stage . This means that , when the projection image plane isset to a position ZC and the surface of the wafer is shifted between thepositions Z₀ and Z₅, the quantity of light of the contact hole imageapplied to the resist layer is substantially constant. In other words,the fact that the quantity of light (the contact hole image) issubstantially constant over the long range in the direction of the opticaxis, means that the apparent depth of focus is made larger to thatextent.

Meanwhile, in the simulation of FIG. 10, the ratio (Z-Span/2·DOF) of themovement range Z-span of the Z stage from the position Z₀ to theposition Z₅ to the "depth of focus" width 2·DOF of the single unit ofthe projection lens is about 2.24. This is because, during exposureoperation from the position Z₀ to the position Z₅, the shutter is fullyopened and the illumination is constant, and actually the ratio isslightly smaller.

From the simulation the optimum value of the movement range of the Zstage can be obtained. If the ratio Z-Span/2·DOF is made smaller fromthe stage shown in FIG. 10, the light quantity distribution IM(Z)rapidly changes to form a Gaussian distribution peaked at the positionZC. If the ratio Z-Span/2·DOF is made larger, the light quantitydistribution IM(Z) falls at the position ZC and has the peaks betweenthe position Z₀ and ZC and between the positions ZC and Z₅. According tothe experiments, it has been formed that, in the case of the contacthole pattern, if the movement range of the Z stage is so determined thatthe light quantity distribution is substantially flat, the apparentdepth of focus is sufficiently extended and further the deterioration ofcontrast is made minimum.

In the third embodiment, a software for carrying out the simulation asshown in FIG. 10 is provided in the stepper, by which the existenceprobability chacteristic curve is produced based on various parametersunder the exposure condition designated by user and the Gaussiandistribution GP(Z) is shifted in the Z direction along the existenceprobability curve, thereby calculating the quantity distribution IM(Z)of the contact hole pattern, where the peak of each Gaussiandistribution is made coincident with the curve of the existenceprobability when the shutter is in the fully opened state, while duringthe opening operation or closing operation of the shutter the peak ofeach Gaussian distribution is adjusted to be on a point lowered from thevalue of the existence probability to the extent of the lowered amountfrom the illumination obtained at the time when the shutter is fullyopened.

The thus obtained light quantity distribution IM(Z) is displayed by acathode-ray tube or the like on the control panel of the stepper, andthe width in the Z direction of the flat portion which is within thetolerance range is calculated and displayed. This is advantageousbecause the operator can confirm previously whether the optimum resultis expected under the set exposure condition, and that resetting ofanother exposure condition can be easily made.

In addition to warning or anouncing as described with respect to thesecond embodiment, anouncing to the operator may be made to notice thatthere is any problem on exposure condition in view of the characteristicof the light quantity distribution IM(Z) obtained by the simulation.

Meanwhile, the substantially constant light quantity distribution in theZ direction as shown in FIG. 10 is particularly advantageous in the casewhere the wafer surface has continuous uneveness since the pattern isformed accurately with respect to each of uneven or stepped portions onthe wafer surface.

In the case where steps or uneven portions are located mainly at thememory cell portion and the peripheral circuit portions as in the caseof DRAM pattern, it is not necessary to make the light quantitydistribution of the contact hole image constant in the Z direction, butrather it may be so set that the light quantity distribution is largerat two portions, the higher portion (memory call portion) and the lowerportion (peripheral circuit) of the stepped portions. For this purpose,the movement range Z-Span of the Z stage may be extendned.

As described above, according to the present invention, during oneexposure time (one shutter opening operation), the projection imageplane and the photosensitive substrate are moved relative to each otherin the direction of the optic axis at a pre-controlled speedcharacteristic and therefore, the effect of enlarging the depth of focuscan be attained without extremely reducing the photosensitive substratetreating ability (throughput) per unit time. Also, the illuminationvariation characteristic of the illuminating light on the mask is foundin advance before the main exposure to the photosensitive substrate andtherefore, the desired exposure amounts (E₁, E₄, etc.) input by theoperator can be exactly concentrated on particular points (two or threepoints) within the focus range set by the operator and at the same time,the total exposure amount for one shot area can also be accuratelycontrolled.

A fourth embodiment of the present invention will now be described.

In this embodiment, as in the previous embodiments, it is a premise thatprovision is made of a projection optical system PL for projecting thepattern of the reticle R onto a predetermined area (a shot area) on thewafer W, an XY stage 13 for two-dimensionally moving the wafer W in aplane perpendicular to the optic axis AX of the projection opticalsystem, spacing changing means (the Z stage 14, the driving portion 15,etc.) for continuously changing the relative spacing between the imagingplane (the best focus plane) of the projection optical system and thewafer W in the direction of the optic axis, and control means (MCU 18D)for operating the spacing changing means in operative association withthe exposing operation of giving a proper exposure amount to a certainpredetermined area.

This embodiment is characterized by the provision of state detectingmeans (steps 208, 210, 212 and 213 by CPU 300) for detecting the stateof the coordination (the timing in terms of time) of the operation ofthe spacing changing means and the exposing operation when apredetermined area to be precedently exposed is exposed under a presetoperational condition, and modifying means (steps 214 and 215 by CPU300) for modifying a parameter which prescribes the operationalconditions (the start timing, speed, etc. of the Z stage) of the controlmeans (MCU 18D) or the condition (the exposure time by the fineadjustment of the intensity of the light source) of the exposingoperation for a predetermined area to be next exposed when the detectedstate is improper.

In the present embodiment, design is made such that when the exposingoperation is repetitively performed substantially under the samecondition, the coordination of the continuous movement of the spacingchanging means (the Z stage) during the preceding exposing operation isanalyzed (learned) to thereby find an error in the coordination and theparameter is modified so that the error may be corrected during thesucceeding exposing operation.

In the present embodiment, design is also made such that for example,the period during which the Z stage is continuously moved in thedirection of the optic axis is accurately set within one exposure time,whereby the weights of the exposure amount become substantially equalmaximum values at two locations spaced apart from each other in thedirection of the optic axis. If this setting is inaccurate, there willarise the problem that the effect of enlarging the depth of focus cannotbe sufficiently enhanced or there occurs a difference in the enlargingeffect for each shot, but in the present embodiment, for example, thecoordination in the shot exposure immediately before is learned and theresult of it is reflected in the next shot exposure and therefore, thestable effect of enlarging the depth of focus can be obtained for almostall shots.

FIG. 11 shows the epitome of a projection exposure apparatus used in thepresent embodiment, and this apparatus is basically the same as theconstruction of FIG. 4. The condenser lens 12 constituting the laststage of the illuminating system applies illuminating light of uniformilluminance to the reticle R on which is depicted a circuit pattern forsemiconductor printing. The reduction projection lens PL of 1/5 or 1/10projects and exposes the pattern of the reticle R onto the wafer W undera both-side telecentric condition. A photosensitive layer (photoresist),not shown, is applied to the wafer W, which is adsorbed onto the Z stage14 including a wafer holder. This Z stage 14 is provided on the XY stage13 for fine movement in the direction of the optic axis AX of theprojection lens PL (Z direction). The Z stage 14 is usually used toalign the surface of the wafer W with the right focus plane (the planeof the projection lens PL which is conjugate with the reticle R), andthe XY stage 13 is two-dimensionally moved to position the projectedimage of the reticle R and the wafer W in a plane perpendicular to theoptic axis AX. The Z stage 14 is movement-controlled with an accuracy ofe.g. 0.1 μm or less so that the surface of the wafer W can be positionedwith higher accuracy within an effective focus range which decreases asthe resolving power (numerical aperture N.A.) of the projection lens PLis improved.

Also, it is known that the optical performances of the projection lensPL such as focus position and magnification (distortion) are fluctuatedby the atmospheric pressure and temperature under the environment of useand the exposure energy power, the quantity of applied light, etc.during the processing of the wafer. Therefore, in the apparatus of thepresent embodiment, a fluctuation amount monitor 50 for inputting theinformation S₁₁ of those environmental conditions and exposureconditions and calculating the fluctuation of the optical performancesof the projection lens PL and a pressure regulator 52 for regulating theair pressure in the projection lens PL in conformity with the calculatedfluctuation amount are provided so as to correct the fluctuation of theoptical performances.

FIG. 12 shows the details of the construction of an automatic focusingmechanism (16, 17, 18) of the oblique incident light type provided inthe projection exposure apparatus shown in FIG. 4, and FIG. 13 is ablock diagram collectively showing the mechanisms of the hardware andsoftware of the control system in the apparatus of FIG. 12. Theautomatic focusing (hereinafter referred to as AF) mechanism will firstbe described with reference to FIG. 12.

Illuminating light LB non-sensitizing to the resist layer on the wafer Wand having a wide wavelength width uniformly illuminates a slit plate16A. The light passed through a slit in the slit plate 16A becomes animaging light beam obliquely incident on the wafer W via a lens system16B, a mirror 16C, a stop 16D, a projection lens 16E and a mirror 16F.Thereby, a one-dimensional slit image is formed at a position on thewafer W through which the optic axis AX of the projection lens PLpasses, i.e., the center of the projected area of the pattern image ofthe reticle R. The above-mentioned members 16A-16F correspond to thelight projector 16 of FIG. 4. The reflected light reflected by the waferW is imaged on a light receiving slit plate 17G via a mirror 17A, anobjective lens 17B, a relay lens 17C, a vibration mirror 17D and planeparallel glass 17F. That is, a light transmitting slit plate 16A, thewafer W and the light receiving slit plate 17G are set so as to becomeconjugate with one another when the wafer W comes to a predeterminedposition (in-focus position) in the direction of the optic axis. Thesystem from the light transmitting slit plate 16A to the light receivingslit plate 17G is disposed without finely moving relative to theprojection lens PL. Assuming here that the wafer W is coincident withthe best imaging plane of the projection lens PL (i.e., is in focus),the slit image on the light receiving slit plate 17G vibrates about theslit in the light receiving slit plate 17G due to the action of thevibration mirror 17D. When the wafer W deviates from the in-focusposition, the center of vibration of the slit image on the slit plate17G becomes displaced from the slit in the slit 17G. A photomultiplier(PMT) 17H photoelectrically detects the quantity of light passed throughthe light receiving slit plate 17G, and the photoelectric signal thereofis output to a phase synchronous demodulator circuit (hereinafterreferred to as PSD) 18B. Also, an oscillator (OSC.) 18A outputs an ACdrive signal of frequency f to an actuator (M-DRV) 17E for driving thevibration mirror 17D and outputs a reference signal of frequency f tothe PSD 18B. The PSD 18B receives as inputs the reference signal fromthe OSC. 18A and the photoelectric signal from the PMT 17H, andsynchronously detects the photoelectric signal relative to the referencesignal. In the above-described construction, the members 17A-17Hcorrespond to the light receiving device 17 in FIG. 4.

This synchronous detection is the same as that effected by a well-knownphotoelectric microscope, and when the center of vibration of the slitimage vibrating on the slit plate 17G coincides with the center of theslit in the slit plate 17G, the photoelectric signal of the PMT 17Hassumes a frequency (2f) just double the frequency f of the oscillationsignal of the OSC. 18A and the detection output FS of the PSD 18Bbecomes zero. When this state is deviated from, the PSD 18B produces adetection output FS differing in polarity, depending on the direction ofthe deviation, i.e., in which direction the position of the wafer W hasbeen displaced relative to the in-focus position.

Accordingly, the detection output FS of the PSD 18B exhibits a such acontinuous voltage change that it is zero in the in-focus state, becomespositive when the wafer W comes, for example, near the projection lensPL side, and becomes negative when the wafer W goes away from theprojection lens PL. The detection output FS is usually called an S curvesignal and near the zero point thereof, there exists a range in whichthe relation between the voltage change and the change in the positionof the wafer in Z direction becomes substantially linear, and this rangeis used for servo control for the movement of the Z stage 14.

Now, the Z stage 14 is moved by a driving portion 15 such as a motorwhich is provided on the XY stage 13 being servo-controlled by a drivingcircuit (Z-DRV) 18C. In the usual mode, the Z-DRV 18C receives thedetection output FS from the PSD 18B as deviation information, anddrives the motor 15 so that the detection output FS may coincide with atarget value DS from a main control unit (MCU) 18D. The vibration mirror17D, M-DRV 17E, the slit plate 17G, the PMT 17H, the OSC. 18A, the PSD18B, the Z-DRV 18C, the motor 15 and the Z stage 14 together constitutea closed loop control system for automatic focusing. Also, an encoderfor detecting the amount of movement of the Z stage 14, or apotentiometer or the like is incorporated in the motor 15 and aninformation signal ES representative of the amount of movement is outputtherefrom.

In FIG. 12, the plane parallel glass (hereinafter referred to as thehalving glass) 17F in the optical path of the AF system is for incliningthe reflected light beam from the wafer W so as to shift in thedirection of vibration of the slit image on the slit plate 17G, and actsas an offset mechanism for shifting the surface to be focused by apredetermined amount in the direction of the optic axis AX.

Now, in the present embodiment, as a method of increasing the apparentdepth of focus of the projection lens PL, the Z stage 14 is continuouslymoved in the direction of the optic axis with a predetermined speedcharacteristic during one exposing operation (during the shutteropening). However, the movement of the Z stage 14 during one exposingoperation is only in one direction from down to up or from up to down.This is for maximally enhancing the throughput.

So, each functional unit in the MCU 18D will hereinafter be describedwith reference to FIG. 13. The MCU 18D is constructed chiefly of acentral processing unit (CPU) 300, and is provided with a timer 301adapted to start time counting in response to a shutter opening commandTG during exposure, a data memory portion 302 storing therein variousparameters for determining the operational condition of the Z stage 14,a digital-analog converter (DAC) 303 for outputting a target value DS tothe Z-DRV 18C, an analog-digital converter (ADC) 304 for converting thelevel of the detection output (S curve signal) FS from the PSD 18B intoa digital value, a position detecting circuit 305 for generating theinformation ES of the amount of movement of the Z stage 14 by the motor15 as a digital value, and a time monitor portion 306 for monitoring thereal time from after the shutter begins to be opened until it iscompletely closed. The CPU 300 exchanges information among the timer301, the memory portion 302, the DAC 303, the ADC 304, the positiondetecting circuit 305 and the time monitor 306 and executes a series ofoperations.

Again in the present embodiment, as in the projection exposure apparatusshown in FIG. 4, a rotary shutter 3 is disposed to change over theinterception and passage of the illuminating light to the reticle R. Therotary shutter 3 is rotated by a motor 4 and the control thereof iseffected by a shutter driving portion 22. The shutter driving portion 22is responsive to the shutter opening command TG to rotate the motor 4 toa position in which the blades of the rotary shutter 3 do not interceptthe illuminating light. When the shutter 3 is opened and theilluminating light arrives at the reticle R, part of the illuminatinglight is also received in a quantity corresponding thereto by aphotoelectric sensor 20. The photoelectric signal of the photoelectricsensor 20 is input to an integrator 21, whereby an integrated valuecorresponding to the exposure amount on the wafer W is calculated. Avalue corresponding to a target exposure amount (a proper exposureamount) is set in a setting portion 45, and the integrator 21 judgeswhether the integrated value coincides with the target value, and whenit coincides with the target value, the integrator 21 outputs a commandfor closing the shutter 3 to a shutter driving portion 22. Thereby,there is provided an automatic exposure control system for making theexposure amount in each shot given to the wafer W substantiallyconstant.

In the above-described construction, the XY stage 13 is moved in thestep and repeat fashion to expose the pattern of the reticle R onto eachof a plurality of shot areas on the wafer W, and when each shot area ispositioned just beneath the projection optical system PL, an exposurestarting command TG is immediately sent from the host computer (the maincontroller 100 in FIG. 4). However, when the alignment of each shot areasuch as die by die alignment or site by site alignment is to be effectedafter the stepping, the exposure starting command TG is sent after thatalignment is completed.

FIG. 14 illustrates the waveform of the photoelectric signal from thephotoelectric sensor 20, i.e., the coordination (timing association) ofthe exposing operation and the position change characteristic of the Zstage 14 in Z direction (the direction of the optic axis AX), and isequal to FIG. 6A. FIG. 14A shows an example of the waveform of thephotoelectric signal from the photoelectric sensor 20, and the ordinateof this figure represents the signal level and the abscissa representstime. The waveform of FIG. 14A primarily corresponds to the variation inthe illuminance on the reticle R or the wafer W. In FIG. 14A, theshutter starting command TG is generated at time T₀, and the shutter 3begins to pass the illuminating light therethrough at time T₁ later bysome lag time (of the order of several milliseconds). The shutterbecomes fully open at time T₂ after the lapse of a substantiallyconstant time (of the order of 10-30 milliseconds), and is stopped in astate in which the signal level is a maximum value ILm. In the meantime,the integrator 21 continues the operation of integrating the level ofthe photoelectric signal, and outputs a closing command to the shutterdriving portion 22 at time T₃ when the target value has been reached. Attime T₄ later by a substantially constant lag time (of the order ofseveral milliseconds), the shutter 3 begins to intercept theilluminating light and at time T₅, the shutter completely intercepts theilluminating light and is stopped. Here, the opening operation time fromthe time T₁ till the time T₂ and the closing operation time from thetime T₄ till the time T₅ become substantially equal values, and eachoperation time and the lag times (T₁ -T₀, T₄ -T₃) become substantiallyconstant values in conformity with the mechanical characteristic,electrical characteristic, etc. of the shutter. The shutter real timemonitor 306 in FIG. 13 outputs to the CPU 300 a digital valuecorresponding to the time from the time T₁ till the time T₅ in FIG. 14A.

FIG. 14B shows an example of the position change characteristic of the Zstage 14 (or the surface of the wafer W) ideally associated with theexposing operation of FIG. 14A in Z direction (the direction of theoptic axis AX), and the ordinate of this figure represents the Zposition and the abscissa represents time. The zero point of the Zposition of FIG. 14B indicates the best focus position detected as thein-focus point by the AF system, and it is to be understood here thatthe Z stage 14 is moved within the range of ±Z₁ about the best focusposition. That is, considering that during one exposing operation, thebest imaging plane of the projection optical system PL exists at theposition of the zero point in FIG. 14B, it follows that relativethereto, the surface of the wafer W has continuously moved in Zdirection from a position -Z₁ to a position +Z₁. In FIG. 14B, at thestart of exposure, the Z stage 14 is at the position -Z₁, and is movedat a velocity Vs from time Ta when the shutter is fully open, andarrives at the position +Z₁ at time Tc and is stopped thereat. Time Tbwhen the surface of the wafer W crosses the best imaging plane (the zeropoint) is substantially intermediate of the times Ta and Tc and is themiddle point of the actually effective exposure time (T₅ -T₁).

When in this manner, during one exposing operation, the Z stage 14 ismoved within the range of ±Z₁ under the timing condition as shown inFIG. 14B, the weight of the exposure amount obtained at each minute Zposition in the direction of the optic axis AX becomes maximum near thepositions -Z₁ and +Z₁ as shown in FIG. 15, and becomes very small at theZ position therebetween. As a result, there is obtained a focusenlarging effect substantially equal to the conventional two-timeexposure system (U.S. Pat. No. 4,869,999). In FIG. 15, the ordinaterepresents the Z position and the abscissa represents the weight ratio(or relative weight) of the exposure amount, and this figure correspondsto FIG. 7 or FIG. 9.

The operation of the present apparatus will now be described withrespect, for example, to a case where exposure is effected for each shotarea (exposed area) on the wafer W in the step and repeat fashion.However, it is to be understood that before the first shot exposure ofthe wafer W, one actually effective exposure time (T₅ -T₁) is foundsubstantially accurately by the real time monitor 306 or by calculation.It is also to be understood that the velocity Vs of the Z stage 14during the exposing operation, the swing width ±Z₁ of the Z stage 14,etc. are stored as initial values in the data memory portion 302. Theswing width ±Z₁ can be suitably designated by the operator.

The CPU 300 of FIG. 13 determines the movement starting time Ta andmovement stopping time Tc of the Z stage 14 at a point of time whereatthe actually effective exposure time (T₅ -T₁), the velocity Vs and theswing width ±Z₁ have been given.

First, the velocity Vs is a value inherent to the Z stage and is usuallyset to the vicinity of the maximum velocity. So, from the absolute value2Z₁ of the swing width and the velocity Vs, the time Tss from the timeTa till the time Tc is found by the calculation of 2Z₁ /Vs. However, thevelocity of the Z stage 14 never becomes linear as per the set value Vsduring the start and stoppage and therefore actually it need be a valuesomewhat greater than the time Tss found by the calculation.

Subsequently, the CPU 300 finds 1/2 of the value of the actuallyeffective exposure time (T₅ -T₁) minus the time Tss. This valuecorresponds to the time Te from the time T₁ till the time Ta (or thetime from the time Tc till the time T₅), and thus the movement startingtiming (time Ta) of the Z stage 14 has been specified. Instead ofspecifying the time Ta with the time T₁ as a reference, the generationtime T₀ of the shutter opening command TG may be used as a reference. Inthat case, the lag time (T₁ -T₀) is added to the time Te.

When the above-described calculation is terminated, the CPU 300preserves the value of the time Te or Te+(T₁ -T₀) as one of initialparameters in the data memory portion 302. At this time, the CPU 300also calculates the time (Te+Tss/2) till the middle point time Tb of theactually effective exposure time (T₅ -T₁), or time (Te+Tss/2+T₁ -T₀) andpreserves it as one of the initial parameters in the memory portion 302.

Now, in the present embodiment, design is made such that the targetvalue DS output from the DAC 303 is varied correspondingly to thepositions ±Z₁ in order to move the Z stage 14 as shown in FIG. 14BB.This will hereinafter be described. FIG. 16 shows an example of thevariation characteristic of the detection output FS from the PSD 18B,and the ordinate of this figure represents voltage (level) and theabscissa represents the amount of deviation of the focus (the amount ofdeviation between the best focus plane and the surface of the wafer) ΔZ.The detection output FS has a range in which the voltage V and theamount of deviation ΔZ become linear about the zero point (the in-focuspoint), and the detection output FS assumes a voltage +V₁ when thesurface of the wafer W is displaced from the in-focus point to theposition +Z₁, and assumes a voltage -V₁ when the surface of the wafer Wis displaced to the position -Z₁.

So, assuming that the target value DS output from the DAC 303 is fixedat -V₁, the servo control of the Z stage 14 is effected so that thesurface of the wafer W may come from the best focus plane to theposition -Z₁.

FIG. 17 shows the construction of the Z-DRV 18C for the motor 15 of theZ stage 14, and a differential amplifier 180 outputs the differencebetween the detection output FS and the target value DS, and adifferential amplifier 181 outputs the difference between the detectionoutput FS and the velocity feedback signal Sv to a power amplifier 182.The motor 15 rotates at a speed conforming to the driving voltage of thepower amplifier 182 and the rotational speed thereof is detected by atachogenerator 183. A feedback circuit 184 applies integrating process,gain correction, etc. to the detection signal from the tacho-generator183 and outputs said detection signal as the feedback signal Sv. Thiscircuit is a speed feedback system for stabilizing the rotational speedduring the driving of the motor 15, and in the present embodiment, it isendowed with the function of reducing the gain to the feedback signal Svfrom the feedback circuit 194 in response to a command CD from the CPU300. That is, it creates a state in which the speed feedback amount hasbeen minimized, thereby heightening the rotational speed of the motor 15and thereby maximizing the movement velocity Vs of the Z stage 14. Thespeed control of the motor 15 is not restricted to that by theconstruction described above, but any control matching the servo systemof the motor 15 can suitably be utilized.

In the construction of FIG. 17, when the target value DS is varied to+V₁ from a state in which the target value DS is zero and the detectionoutput FS is also stable at the zero point, the motor 15 begins torotate at a high speed from that moment and the Z stage 14 is moved fromthe zero point to the position +Z₁. When the Z stage 14 comes to theposition +Z₁, the motor speed becomes stabilized at DS=FS=+V₁ and thesystem thereof is stabilized. Accordingly, the swing width of the Zstage 14 in the present embodiment is limited to a linear range in termsof the characteristic of the detection output FS, but this is a rangesufficient to cope with because the optimum swing width for obtainingthe effect of enlarging the depth of focus is considered to be of theorder of the width of the depth of focus of the projection opticalsystem (e.g. ±μ1 m). When it is desired to make the swing width of the Zstage 14 greater than that, the inclination of the halving glass 17Fshown in FIG. 12 can be continuously changed as indicated by thecharacteristic of FIG. 14B during the movement period of the Z stage 14,i.e., during the time Ta to the time Tc in FIG. 14B and also, the Zstage 14 (the motor 15) can be servo-controlled so that the zero point(or a predetermined voltage point) of the detection output FS may alwaysbe obtained.

Now, exposure of the step and repeat type to a wafer W is executed inaccordance with flow charts shown in FIGS. 18 and 19. These flow chartsare executed chiefly by the CPU 300 and have portions which shouldoriginally be processed by the interrupt system, but are shown in theform of a single routine for the ease of understanding.

First, the host computer (the main controller 100 of FIG. 4) designatesthe stepping coordinates to bring the position of the XY stage 13 tosuch coordinates that the first shot on the wafer W is exposed. Thereby,the XY stage 13 is stepped so that the first shot may come to theposition of the projected image from the projection optical system PL(step 200). At this time, the CPU 300 outputs to the DAC 303 a digitalvalue which will render the same value as the level of the detectionoutput FS corresponding to the position -Z₁, i.e., the voltage -V₁, intothe target value DS on the basis of the value of the swing width set inthe data memory portion 302 (step 201). Thereby, the Z stage 14 isservo-locked at such a position that the surface of the wafer isdisplaced by -Z₁ relative to the focus plane.

When the positioning of the XY stage 13 and the Z stage 14 is completedand these stages become stationary, at a step 202, the shutter opening(exposure starting) command TG is generated and the driving of the motor4 for the shutter 3 is started and at the same time, thequantity-of-light integrating operation of the integrator 21 and thetime counting of the timer 301 are started. Thereafter, at a step 203,the CPU 300 reads the time count value of the timer 301, and at a step204, it judges whether that value has reached the driving start time Taof the Z stage 14. As previously described with reference to FIGS. 14Aand 14B, the time from after the starting command TG has been generatedtill the start time Ta is set as Te+(T₁ -T₀) in the memory portion 302and therefore, at the step 204, the CPU 300 judges whether the timecount value of the timer 301 has become Te+(T₁ -T₀). In the meantime,the shutter real time monitor 306 starts time counting from the time T₁when the shutter 14 begins to pass the illuminating light therethrough.

When it is judged that the start time Ta of the Z stage 14 has beenreached, at a step 205, the CPU 300 outputs to the DAC 303 a digitalvalue for rendering the target value DS into a voltage +V₁ correspondingto the position +Z₁ of the Z stage 14. Along therewith, the Z-DRV 18Cshown in FIG. 12 starts to move the Z stage 14 substantially at thehighest velocity from the position -Z₁ toward the position +Z₁.Immediately thereafter, the CPU 300 reads the level of the detectionoutput FS of the PSD 18B through the ADC 304 (step 206), and judges at astep 207 whether that level has become substantially zero. The detectionof whether the detection output FS has become substantially zero will bemore realistic if it is effected by passing the output FS through a zerodetecting comparator of narrow window width or the like and applyinginterruption to the CPU 300 in response to the zero detection signal(pulse) thereof.

Now, immediately after the Z stage 14 has started its movement towardthe position +Z₁, the detection output FS has not yet reached the zeropoint and therefore, the judgment at the step 207 is "No", and the CPU300 advances the sequence to a step 209. Also, if it is judged at thestep 207 that the detection output FS is zero, the CPU 300 reads, at astep 208, the time count value Tmc of the timer 301 and stores it intothe data memory portion 302. This time count value Tmc is used to checkup later whether the time Tb in FIG. 14B has coincided with the middlepoint of the real exposure time (T₅ -T₁). After the step 208, the CPU300 returns the sequence to the step 206. The condition branching-offfrom the step 207 to the step 208 may be limited to once or may not belimited at all.

Thus, the loop of the steps 206, 207 and 209 is executed until the Zstage 14 arrives at the position +Z₁ after the detection output FS hascrossed the zero point, and when the level of the detection output FSbecomes substantially equal to the target value DS (+V₁) set at the step205, at a step 210, the CPU 300 reads the then time count value Tme ofthe timer 301 and stores it into the memory portion 302. Thereafter, ata step 211, the CPU 300 waits for the termination of the exposingoperation for the first shot. The read time count value Tme is used tocheck up the termination time Tc of the movement of the Z stage 14 inFIG. 14B.

When the exposure of the first shot is thus terminated, at the step 212of FIG. 19, the CPU 300 reads the real exposure time (T₅ -T₁) from thereal time monitor 306 and stores it into the memory portion 302. At thenext step 213, the CPU 300 analyzes an error in the coordination (timingrelationship) of the exposing operation by the driving of the shutter 3and the movement control of the Z stage 14. The CPU 300 first subtractsthe movement time of the Z stage 14, i.e., the difference time Tss'between the time count value Tme and the time count value (Te+T₁ +T₀),from the real exposure time (T₅ -T₁) for the first shot. Further, theCPU 300 finds the value Te' of 1/2 of the time obtained by subtractingthe time Tss' from the real exposure time (T₅ -T₁). If there is noerror, this time Te' becomes equal to the time (Ta-T₁) and the time (T₅-Tc) in FIG. 14B. However, with regard particularly to the first shot,there is a great possibility that the values of those three, i.e., thetime Te', the time (T₅ -T_(c)) and the time (Ta-T₁) become greatlydifferent from one another.

So, in the present embodiment, the error ΔTe' between the time Te' andthe time (T₅ -Tc) is calculated and whether it is equal to or greaterthan an allowable amount. Here, the time (T₅ -Tc) can be found byfurther subtracting the value of the time count value Tme read at thestep 210 when the Z stage 14 has come to the position +Z₁, minus the lagtime (T₁ -Tb) during the start of shutter opening from the real exposuretime (T₅ -T₁) read by the real time monitor 306. When the error ΔTe' isequal to or greater than the allowable value, at a step 214, the CPU 300judges that the error should be corrected, and at a step 215, itcorrects the parameter in the memory portion 302. Specifically, the time(Te+T₁ -T₀) till the movement start of the Z stage 14 set before theexposure of the first shot can be renewed to a time (Te+T₁ -T₀ -ΔTe')corrected by the error ΔTe'.

As another correcting method, it is conceivable to judge whether 1/2 ofthe real exposure time (T₅ -T₁) of the first shot and the time countvalue Tmc read at the step 208 minus the lag time (T₁ -T₀) aresubstantially equal to each other, and apply a correction to the time(Te+T₁ -T₀) as the error ΔTe' when the error therebetween is equal to orgreater than the allowable value.

Subsequently, at a step 216, the CPU 300 judges whether the exposingoperation for all shots on the wafer W has been terminated, and if saidexposing operation has not been terminated, the CPU 300 again executesthe sequences from the stem 200 of FIG. 18. The series of sequences ofthe above-described steps 200 to 216 are repeated for each shot on thewafer W, and the accuracy of the coordination of the operation of theshutter 3 (the shot exposing operation) and the movement of the Z stage14 is enhanced more in the second shot than in the first shot, more inthe third shot than in the second shot, and so on. That is, thecoordination (timing relationship) learned during the shot exposureimmediately before is always reflected in the coordination controlduring the subsequent shot exposure.

As described above, in the fourth embodiment, design is made such thatthe timing of the shutter operation and the movement of the Z stage interms of time is analyzed to optimize the coordination, but besides, theweight ratio of exposure amount as shown in FIG. 15 can be analyzed tooptimize the coordination. As regards also the parameter correction, notonly time but also the preset movement velocity Vs of the Z stage 14 canbe delicately varied.

Further in the fourth embodiment, the time Tmc during which the surfaceof the wafer crosses the best focus plane is monitored and therefore,the velocity irregularity of the Z stage 14 can also be specified to acertain degree, whereby such parameter correction that will control thevelocity characteristic is also possible. Considering the time countingby the timer 301 as a reference, the Z stage 14 is started after thelapse of a time (Te+TL) when the lag time (T₁ -T₀) is regarded as timeTL, crosses the zero point at the time Tmc, and is stopped at the timeTme.

Therefore, ideally, it ought to follow that (Te+TL+Tme)/2=Tmc. However,if there is a difference between the velocity change during the start ofthe Z stage 14 (the rising characteristic) and the velocity changeduring the stoppage of the Z stage 14 (the falling characteristic), itfollows that the symmetry thereof is greatly destroyed. So, the symmetryof the velocity characteristic can be found about the time when the Zstage 14 crosses the zero point, and the velocity control of the Z stage14 can be adjusted if the symmetry is bad. Specifically, such a commandCD which will vary the velocity feedback amount by the feedback circuit184 as shown in FIG. 17 between the movement starting point to thestopping point of the Z stage 14 can be given.

Now, in the above -described embodiment, the depth-of-focus enlargingeffect obtained at the first shot for the wafer W cannot always be saidto be stable. Therefore, the pattern resolution in the first shotbecomes insufficient and it may happen that the wafer becomes inferioras a device chip.

So, as shown in FIG. 20, of the shot areas arranged on the wafer W, thefirst shot is always designated so as to expose a shot SA₁ which will bepartly broken on the marginal edge of the wafer W. The exposure to thisbroken shot SA₁ is a kind of dummy exposure. In FIG. 20, arrows likingthe centers of respective squares represent the direction of stepping bythe XY stage 13. Also, when it is desired to expose the resist on themarginal edge portion of the wafer as well, stepping is effected so asto expose all broken shots existing on the marginal edge portion aswell. So, the correction of the parameter may be effected during theexposure of the broken shot. In this case, as long as normal shots areexposed, the correction of the parameter is not effected and the steps212-215 of FIG. 19 are executed by the last one of normal shotscontinuously exposed to thereby effect the correction of the parameter.

The correction of the parameter is possible not only by the adjustmentof the velocity and driving timing of the Z stage, but also by theadjustment of the opening time of the shutter 3. In that case, theintensity of the illuminating light on the reticle R when the shutter isfully open is changed to thereby change the real exposure time (T₅ -T₁)for obtaining a proper exposure amount. Generally the light source usedin an apparatus of this kind is a mercury discharge lamp and therefore,the intensity of the illuminating light can be adjusted by varying theinput electric power to the discharge lamp. As a case where thecorrection of the parameter for changing the real exposure time (T₅ -T₁)is effective, mention may be made of a case where high sensitivityresist or the like is used and the fully opening time of the shutterbecomes very short. If in this case, the coordination of the movementcharacteristic of the Z stage 14 and the operational characteristic ofthe shutter deviates slightly, the balance of the weight ratio of theexposure amount at the Z positions ±Z₁ may sometimes differ greatly. So,the input electric power to the discharge lamp can be reduced slightly(e.g. by the order of 10-30%) to thereby lengthen the fully opening timeof the shutter.

Further, in the above-described fourth embodiment, during one exposingoperation, the Z stage 14 is moved in the direction of the optic axis,that is, the wafer is moved in the direction of the optic axis relativeto the best imaging plane of the projection lens PL, but alternatively,the wafer may be fixed and the best imaging plane may be moved in thedirection of the optic axis. Specifically, offset which will vary by apredetermined amount during one exposing operation can be applied to thecommand value S₁₂ to the pressure regulator 52 shown in FIG. 11. If thisis done, the focus position of the projection lens PL itself (theposition of the best imaging plane conjugate with the reticle R) willvary by a minute amount and therefore, an effect equal to that of theprevious embodiment will be obtained without involving any mechanicaldriving. However, the time variation characteristic of the pressureoffset to be applied during one exposing operation has a tendencysimilar to that shown in FIG. 14B.

Also, as a mechanical driving system, the reticle R can be moved in thedirection of the optic axis or the lens element in the projection lensPL can be moved in the direction of the optic axis. Particularly wherethe reticle R is moved, the movement is enlarged to square times (25times or 100 times) of the inverse number of the projectionmagnification (1/5 or 1/10) relative to the swing width ±Z₁ when thewafer W is moved, and this leads to the advantage that mechanicaldriving control becomes easy.

In the fourth embodiment, as shown in FIGS. 14A and 14B, the Z stage 14is moved with a velocity characteristic symmetrical with respect to themiddle point (time Tb) of the real exposure time (T₅ -T₁), but settingneed not always be done so that at the middle point of the real exposuretime, the surface of the wafer may cross the best focus plane. In thatcase, the weight ratio of the exposure amount shown in FIG. 15 willassume different magnitudes at the Z positions +Z₁ and -Z₁. Suchweighting of the exposure amount copes with being suitably changed bythe operation depending on the thickness of the resist layer to beexposed, the structure of the resist layer, the material of groundwork,etc.

Also, where of the shutter characteristics shown in FIG. 14A, theopening operation time (T₂ -T₁) and the closing operation time (T₅ -T₄)differ from each other, even if the weights of the exposure amounts atthe Z position +Z₁ are set equally, the middle point of the realexposure time (T₅ -T₁) and the timing at which the surface of the wafercrosses the best focus plane will somewhat deviate from each other. Thistendency will become remarkable particularly when the fully opening time(T₄ -T₂) of the shutter becomes relatively short.

Thus, according to the fourth embodiment, a substantially stabledepth-of-focus enlarging effect is obtained during each shot in whichexposure is continuously effected onto the photosensitive substrate.Further, even if the intensity of the illuminating light for exposurefluctuates slightly while a photosensitive substrate is exposed, wherebythe exposure time per shot is varied, the parameter is optimallycorrected with that taken into account and therefore, exposure in whichthe weight ratio of the exposure amounts obtained at least two Zpositions in the direction of the optic axis of the projection opticalsystem is kept exactly constant becomes possible. Further, during oneexposing operation, the Z stage is moved only in one direction and thus,there is obtained the effect that the throughput is higher than in theprior art. The fourth embodiment can also be applied to a stepper usingan excimer laser or the like as a light source. In such case, thecontrol of exposure amount by the shutter is not effected, yet theperiod of a plurality of (20-200) timings of the oscillation triggerpulse signal for the light emission of the excimer laser source and theoperation timing of the Z stage can be monitored.

What is claimed is:
 1. A projection exposure apparatus comprising:anilluminating system for illuminating a mask formed with a predeterminedpattern; a shutter for changing over the supplying of illumination lightto said mask and the intercepting of the illumination light; aprojection optical system for projecting the pattern image of said mask;a movable stage holding a photosensitive substrate near the projectionimage plane of said projection optical system and capable of moving saidphotosensitive substrate in the direction of the optic axis while saidshutter is open; stage control means for controlling said movable stagewith a predetermined operational characteristic; shutter control meansfor controlling the opening-closing of said shutter on the basis of thequantity of the illumination light to said mask; and interlocking meansfor interlocking the operations of said shutter control means and saidstage control means so that on the basis of the operationalcharacteristic of said shutter and the operational characteristic ofsaid movable stage, the distribution of an exposure amount provided tosaid photosensitive substrate from the opening operation starting pointof time till the closing operation completing point of time of saidshutter, with respect to the direction of the optic axis, may assumesubstantially equal maximum values at least two locations in thedirection of the optic axis.
 2. A projection exposure apparatusaccording to claim 1, wherein said shutter has such an operationalcharacteristic that the opening operation time from the openingoperation starting point of time till the opening operation completingpoint of time and the closing operation time from the closing operationstarting point of time till the closing operation completing point oftime become substantially equal to each other, and said interlockingmeans includes a speed interlocking control portion for setting themovement speed of said movable stage to substantially equal first valuesat the opening operation starting point of time and the closingoperation completing point of time of said shutter, and setting themovement speed of said movable stage to a second value greater than saidfirst values in the section existing in the period from the openingoperation completing point of time till the closing operation startingpoint of time of said shutter.
 3. A projection exposure apparatusaccording to claim 2, wherein said speed interlocking control portionsets the movement speed of said movable stage so that the time from saidopening operation starting point of time till starting of said secondvalue of the movement and the time from the termination of said secondvalue of the movement till said closing operation completing point oftime are substantially equal to each other.
 4. A projection exposureapparatus according to claim 2, wherein said interlocking means includesa calculating portion for calculating an exposure time from said openingoperation starting point of time till said closing operation completingpoint of time necessary to provide proper exposure to saidphotosensitive substrate, on the basis of the value detected by aphotoelectric detector, and a judging portion for judging whether thereis a difference between said exposure time and the time necessary forsaid movable stage to execute a speed variation determined by said speedinterlocking control portion, and outputting a warning signalrepresentative of becoming non- interlocking when said exposure time isshorter than said necessary time.
 5. A projection exposure apparatusaccording to claim 4, wherein said interlocking means includes anillumination intensity regulating portion for regulating the intensityof illuminating light to said mask in conformity with said warningsignal.
 6. A projection exposure apparatus which has an illuminatingsystem for illuminating a mask formed with a predetermined pattern, aprojection optical system for projecting the pattern image of said mask,and a movable stage holding a photosensitive substrate to be exposed bysaid pattern image, near a projection image plane of said projectionoptical system, said apparatus comprising:(a) illumination means forcontrolling start and end of illuminating said mask; (b) stage drivemeans for driving said movable stage to move said photosensitivesubstrate in the direction of the optical axis of said projectionoptical system; and (c) velocity control means for outputting to saidstage drive means such velocity control signals that a velocity of saidmovable stage takes a first value between a time point at the start ofsaid illuminating and a predetermined time point prior to the end ofsaid illuminating and takes a second value, which is larger than saidfirst value, after said predetermined time point has been reached.
 7. Aprojection exposure apparatus according to claim 6, wherein saidillumination control means and said velocity control means are sosynchronized that a surface of said photosensitive substrate may crosssaid projection image plane in one direction in a state where thevelocity of said movable stage has taken said second value.
 8. Aprojection exposure apparatus provided with a projection optical systemfor projecting the pattern of a mask onto a predetermined area on aphotosensitive substrate, a two-dimensional stage for holding saidphotosensitive substrate and two-dimensionally moving it in a planesubstantially perpendicular to the optic axis of said projection opticalsystem to thereby successively expose different predetermined areas onsaid photosensitive substrate, spacing changing means for continuouslychanging the relative spacing between the imaging plane of saidprojection optical system and said photosensitive substrate with respectto the direction of said optic axis, and control means for operatingsaid spacing changing means in operative association with an exposingoperation for giving a proper exposure amount to said predeterminedareas, characterized by the provision of:state detecting means fordetecting the state of the coordination of the operation of said spacingchanging means and said exposing operation when a predetermined area onsaid photosensitive substrate which is to be precedently exposed isexposed under a preset operational condition; and correcting means forcorrecting a parameter prescribing the operational condition of saidcontrol means or a parameter prescribing the condition of the exposingoperation, for a predetermined area exposed after said precedentpredetermined area, when said detected state is improper.
 9. Aprojection exposure apparatus according to claim 8, further providedwith a light source generating illuminating light for exposure to saidmask, a shutter for changing over the passage and interception of saidilluminating light, a photoelectric sensor for receiving part of theilluminating light passed through said shutter and outputting a signalconforming to the intensity thereof, and shutter control means fordetecting whether said proper exposure amount is given, and outputting asignal for closing said shutter when said proper exposure amount isreached.
 10. A projection exposure apparatus according to claim 9,wherein said state detecting means includes a shutter monitor circuitfor detecting the lapse of the operation time from the start of theopening of said shutter till the termination of the closing of saidshutter, a circuit for detecting the operation timing of said spacingchanging means correspondingly to the lapse of the time detected by saidshutter monitor circuit, and a circuit for analyzing an error betweenthe lapse of the operation time of said shutter and the operation timingof said spacing changing means.
 11. A projection exposure apparatusaccording to claim 10, wherein said correcting means includes a timingcorrecting circuit for staggering the operation timing of said spacingchanging means within the period of the operation of said shutter whenthe error found by said error analyzing circuit is equal to or greaterthan an allowable value.
 12. A projection exposure apparatus accordingto claim 10, wherein said correcting means includes a speed adjustingcircuit for adjusting the changing speed of said spacing changing meanswhen the error found by said error analyzing circuit is equal to orgreater than an allowable value.
 13. A projection exposure apparatusaccording to claim 10, wherein said correcting means includes a circuitfor changing the intensity of the emitted light of said light source tothereby adjust the time from the start of the opening of said shuttertill the termination of the closing of said shutter when the error foundby said error analyzing circuit is equal to or greater than an allowablevalue.
 14. A projection exposure apparatus according to claim 8, whereinsaid state detecting means includes a circuit for analyzing the error ofthe operation timing of said spacing changing means relative to thelapse of the time of said exposing operation, and said correcting meansincludes a timing correcting circuit for staggering the operation timingof said spacing changing means when the error found by said erroranalyzing circuit is equal to or greater than an allowable value.